The United States has castigated Israel for its "totally unacceptable and totally indefensible" shelling of a United Nations shelter in the Gaza Strip.

"The shelling of a UN facility that is housing innocent civilians who are fleeing violence is totally unacceptable and totally indefensible," White House spokesman Josh Earnest said.

Earnest also said that there was little doubt that the shells which struck the UN shelter were fired by Israel.

The most powerful country in the world called on Israel to do more to protect civilian lives as the casualties in the territory were "too high".

The United States represents one of Israel's closest allies, giving over $3bn in foreign aid (25% of US' foreign aid budget) to the Israeli military, according to a US Congressional Research Service report.

The UN also condemned Israel over the worsening humanitarian situation and said that people were "facing a precipice".

"The reality of Gaza today is that no place is safe," UN under-secretary-general for humanitarian affairs Baroness Valerie Amos told the Security Council.

International opinion has begun to round on Israel with Bolivian President Evo Morales declaring Israel a "terrorist state" while El Salvador has joined four other Latin American countries - Brazil, Chile, Ecuador and Peru - in recalling its ambassador from Tel Aviv in protest at the Gaza offensive.

Israeli Prime Minister Benjamin Netanyahu had earlier stated that he would not end the ground and air operation on the territory until all of Hamas' tunnels had been located and destroyed.

A senior Israeli official has claimed that the Israeli military have "neutralised" 70-80% of Hamas' tunnel network.

The UN has estimated that 225,178 Palestinians are now taking refuge in 86 shelters while the Gaza health ministry said that over 1,400 Palestinians had now been killed in the conflict, 315 of those children.

The Israeli military has confirmed the deaths of 56 soldiers while two Israeli civilians and one Thai national have also been killed in the conflict.